1. Field of the Invention
The present invention relates to an optical measuring method and an optical measuring apparatus for making possible the precise measurement of the quality of printed images that are output in an image forming apparatus (image output apparatus) such a color printer. The present invention also relates to an image forming apparatus which has a built-in optical measuring apparatus and is capable of image quality control by feeding back the measured results.
2. Description of the Related Art
With the progress of network technology centering on computers, network printers, for example, printers as image forming apparatus connected to networks have come into wide use rapidly. As the outputting of color images becomes popular, color printers are being increasingly developed now and so is a demand for improvement in the stabilization and uniformization of color image quality among a plurality of color printers.
In order to deal with such a demand, there arises the necessity of measuring the image quality of images that have been output, particularly color difference in the case of color images, and feeding back the measured results to each process step of forming images. Importance has specifically been attached to technology of using printers having built-in measuring apparatus so as to monitor the quality of output images on-line and to feed back the measured results. Consequently, a growing demand is for precision, low-cost, though small-sized, measuring apparatus to be developed as those fit for use in printers.
As far as a image quality monitor is concerned, a measuring apparatus called an X-rite capable of precise image measurement is now in wide use as what is used to make measurement off-line in such a state that an image forming medium such as paper is fixed.
However, unlike the aforementioned X-rite, no on-line precision color measuring apparatus has been put to practical use yet. This is due to the fact that precise measurement is impossible as it has posed a serious problem that in the case of on-line color measurement, the paper surface of paper as an image forming medium to be examined or an object of measurement is caused to undergo vertically fluctuating movement, that is, fluctuating movement in a direction perpendicular to the direction in which the paper is moved forward by a conveyer system.
When an optical system comprising a light source, a lens and a light reception element (photoelectric conversion element) in combination is used as used in the aforementioned X-rite to examine the paper surface making vertically fluctuating movement, for example, when the paper surface makes a vertically fluctuating movement of about 1 mm, the quantity of light received by the light reception element fluctuates because of reflection and scattering on the paper surface. Therefore, the output of the light reception element varies by about 15% and causes a great error to the output of the light reception element, thus rendering infeasible precise color measurement.
There has also been proposed a measuring apparatus for measuring colors on-line by correcting the vertically fluctuating movement of the paper surface like that. This apparatus is, as shown in FIG. 20, constituted of a characteristic measuring unit 1, a distance measuring unit 2 and a distance correction calculating unit 3.
The characteristic measuring unit 1 employs a color measuring sensor in order to measure the colors of a color image formed on paper 4 when the paper 4 as an object of measurement is conveyed in a direction of an arrow and supplies the measured color output to the distance correction calculating unit 3.
The distance measuring unit 2 is provided backward from the characteristic measuring unit 1 in the direction in which the paper 4 is conveyed and employs a distance detection sensor in order to measure the vertically fluctuating movement of the paper 4 at that position. Then the distance measuring unit 2 supplies the output of the measured vertically fluctuating movement to the distance correction calculating unit 3. The distance D between the characteristic measuring unit 1 and the distance measuring unit 2 is 70 mm, for example.
The distance correction calculating unit 3 corrects the measured color output from the characteristic measuring unit 1 using the output of the measured vertically fluctuating movement of the paper 4 from the distance measuring unit 2 and eliminates the influence of the vertically fluctuating movement of the paper 4. Further, the distance correction calculating unit 3 outputs the corrected measured color output as the measured result.
On the other hand, Japanese Patent Laid-Open No. 16247/1988 discloses a diffusion reflectance measuring apparatus which is said to be substantially free from the influence of the distance between the measuring apparatus and a sample (object of measurement) as long as the measured results are within a predetermined range. This apparatus is designed to accomplish the object by uniformizing the intensity of illuminating light on a surface to be examined. FIG. 21 shows a schematic construction of this measuring apparatus.
As shown in FIG. 21, the measuring apparatus above employs a point source 11 as a light source, which is placed at the focal position of a condenser lens 12, whereby rays of light emitted from the point source 11 become parallel rays of light by means of the condenser lens 12 and are incident on the paper surface 10b of paper conveyed in the direction of an arrow 10a. At this time, a range W of light illumination on the paper surface 10b is set greater than a measuring range m. Then the light reflected from the measuring range m on the paper surface 10b is received by the edge face 14a of an optical fiber 14.
The method described in the above publication is intended to maintain the illumination intensity substantially constantly in the measuring range m smaller than the range W of light illumination by making the parallel rays of light incident with respect to the paper surface 10b, irrespective of the distance between the point source 11 and the edge face 14a of the optical fiber. By maintaining the illumination intensity constantly, the measured result is set substantially free from the influence of the distance between the paper surface 10b and the edge face 14a of the optical fiber even the movement of the paper surface 10b vertically fluctuates within a predetermined range .apprxeq.d.
In the case of such a measuring apparatus of FIG. 20, color and distance measurement at each place intended for measurement cannot be made simultaneously because the characteristic measuring unit 1 and the distance measuring unit 2 have to be separated by the distance D from each other in the direction in which the paper is conveyed. Consequently, precise correction is hardly made and this results in greatly restricting the possibility of increasing the precision of the measured color output.
In view of the construction of the apparatus, since the characteristic measuring unit 1 and the distance measuring unit 2 together with the correcting calculation are essential, the shortcoming is that the apparatus tends to become large-scale and costly.
The method as disclosed in Japanese Patent Laid-Open No. 16247/1988 and shown in FIG. 21 uses the point source 11 as a light source and the condenser lens 12 for forming parallel rays of light so as to maintain the illumination intensity substantially constantly on the measuring paper surface 10b, whereby to obtain the measured result unaffected by the vertically fluctuating movement of the paper surface 10b. However, the originally nonexistent of a perfect point source results in making unavailable not only the uniform quantity of light but also parallel rays of light even though the point source 11 is placed at the focal position of the condenser lens 12. For this reason, the illumination intensity on the paper surface 10b is caused to vary by the vertically fluctuating movement of the paper surface 10b.
Since it has also been arranged that the rays of light reflected from the paper surface 10b are focussed into an image on the edge face 14a of the optical fiber via the lens 13, the image forming point deviates from the edge face 14a of the optical fiber likewise when the position of the reflection point as the movement of the paper surface 10b vertically fluctuates. After all, this method described in the above publication renders hardly obtainable the measured result unaffected by the vertically fluctuating movement of the paper surface 10b.
In addition to the aforementioned method, it is also considered feasible as one of the general measures taken to suppress the vertically fluctuating movement of a paper surface to press down paper by means of rollers. However, there are physical limitations to pressing down the paper strongly because the toner image formed on the paper surface has to be prevented from being peeled off or deviated. Even if the paper is allowed to be pressed down strongly, the vertically fluctuating movement of the paper would be impossible to decrease to zero; in other words, at least about several 100 .mu.m still remain and consequently the output of the light reception element varies, thus making a precise measured output unobtainable.